Remove timestamp from local build number

The build_info.prop file was converted to soong, which then caused it to
have a stale build number due to not adding the build number file as a
dependency to avoid rebuilds. I added it as a dependency to fix the
staleness, but it always rebuilds locally now.

I think the way it worked before, it would get stale build numbers,
except that it was cleared with installclean which was always run on CI.
Now that it's a soong module and generated in out/soong/.intermediates
it's not cleared by installclean. I could make a system to register
files that should be installclean'd from a soong module, but I'd
eventually like to eliminate the need to run installclean entirely.

So as an alternative, just make the build number not change every build
when doing local builds, by removing the timestamp from it.

Bug: 346757289
Test: m repeatedly and observe no rebuilds the second time (of the system image, the build flags infrastructure seems to do some small rebuilding)
Change-Id: I0207feb739523dde3e89d1e5c4822865f641c313